probably 95% of the sales pages out there are not full width!
Maybe a narrower page allows better focus and therefore conversion. Reading long lines of text is not easy and very distracting. It is like reading a very long sentence or a very long paragraph.
Have you ever tried to read a 100 lines paragraph?
Not easy, right?
well, same goes for sales pages.
If the goal is sequential reading and focus, then it is easier to narrow the field of attention to something like 700pix